The invention relates to a VCSEL, vertical cavity surface emitting laser.
A VCSEL diode is an injection diode laser where the laser oscillation and output occur perpendicular to the pn junction plane: "Surface Emitting Semiconductor Lasers", Kenichi Iga et el, IEEE Journal of Quantum Electronics, Vol. 24, No. 9, Sep. 1988, pp. 1845-1854; and "Lasing Characteristics of GaAs Microstructures", J. L. Jewell et el, Applied Physics Letters, Vol. 54, No. 15, 10 Apr. 1989, pp. 1400-1402. Desirable properties include low divergence circular longitudinal axial output, and high two-dimensional packing density for arrays, which make VCSELs attractive for applications such as optical recording, communications, and computing.
A drawback of VCSELs is the relatively large injection current required to reach lasing threshold. A VCSEL emits laser light axially vertically in a direction perpendicular to the pn junction plane. This lasing mode light is also known as the stimulated emission and is emitted into an axial normal mode. A VCSEL also emits light laterally horizontally along a direction parallel to the pn junction plane. The lateral horizontal light has two types of modes, namely: the lateral normal mode for which the emitted light is trapped within the cavity; and the lateral traveling mode, also known as a leaky mode, for which the emitted light escapes the cavity. At low levels of forward bias diode injection current, the lateral traveling modes dominate, and significantly more light is emitted laterally than axially. The lateral light is not coherent laser light. It is not until higher levels of injection current that stimulated emission into an axial normal mode becomes dominant.
In the present invention, lasing threshold is reduced by suppressing spontaneous emission into lateral traveling modes such that stimulated emission into axial normal modes become dominant at lower injection currents, thus lowering lasing threshold.
The present invention provides a simple and effective method and structure for lowering lasing threshold. A particularly desirable advantage of the invention is that it enables usage of a conventional VCSEL epitaxial crystal structure diode, and requires only a simple modification to lower lasing threshold.